Radar ground-controlled approach system for aircraft



3 Sheets-Sheet l C. W. SHERWIN ETAL RADAR GROUND-CONTROLLED APPROACHSYSTEM FOR AIRCRAFT Feb. 12, 1952 Filed Aug. 11, 1944 www MMS www ef mwM El Mw M ME LDH #www eww Febi2 952 c. w. SHERWIN ETAL 2,585,855

RADAR GRouND-coNTRoLLED APPROACH SYSTEM RoR AIRCRAFT Filed Aug. 1l, 19443 Sheets-Sheet 2 l0 MILE 2 MIL E E l f 57/ WE'EP SWEEP SWEEP i 'Am-71...I

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Feb. l2, 1952 5 Sheets-Sheet v1';

Filed Aug. 1l, 1944 N lll N www T Mw/L e m WWJQH f 3 ISH a n. Rm ,MH nMM ,4 2 AW mm B o 2 W l m .H t) w .m n n mlm um m1 w J 2 2 m a LTI riff.2 u n n 4 ,f a f n Ae u M m O M No liv ./o M F H L l 9 H /0 W M f j A 2x O ww P P. M im ma MM n @7 01 m m Wl l M l l W N 0 w Mw n w. l M l a mil I f A v Mx i Patented Feb. i2, i952 SATES PATENT OFFICE RADARGROUND-CONTROLLED SYSTEM FOR AIRCRAFT Application August 1l, 1944,Serial No. 549,044

18 Claims.

This invention relates to a communication system and particularly to asystem for controlling landing of aircraft. The invention hereinafterdisclosed and claimed is an improvement upon the invention disclosed andclaimed in the copending application of Luis W. Alvarez and LawrenceJohnston, Serial No. 523,878, led Februar; 25, 1944, Patent No.2,555,101, dated May 29, 1951. As disclosed insaid application. radarsystems disposed at landing areas (either on land or on carriers) areprovided and adapted to control and guide an aircraft during a blindlanding or approach. The system disclosed and claimed in. saidapplication provides means whereby an idealized landing path ispresented for comparison with the actual landing or approach path ofaplane in ight. Thus suitable control signals may be communicated to theFig. 6 is a sectional View of the absorber unit for the antenna system;

Fig. '7 is a perspective showing the dipoles mounted in the wave guide;

Figure 8 is an elevation perspective of the variable wave guide used inthe antenna system;

Fig. 9 is a sectional view showing certain portions of the Variable waveguide;

Fig. l0 shows the aural unit; and

Figs. 11 and 12 show the screens of the indicater tubes under certainconditions.

General description of the system The system herein is broadly dividedinto two portions, a search system and the control system proper. IThesearch vsystem for the most part is a conventional radar search systemhaving a substantial range and adapted to scan a substantial region.Preferably the search system is adapted to`scan the entire 360 inazimuth and may have any desired elevation characteristlcs. Thus formany purposes, the elevation range of the system may be of the order ofabout 10 or 15 above horizontal so that for all practical purposessubstantially complete search coverage may be provided for incomingplanes. The search system may present its data in any sultable fashionand, as shown. may present the rata on the screen of a cathode ray tubewith the presentation being polar. In this form of presentation, theradius vector corresponds to range. while the angle corresponds toazimuth. me search system itself is not independent oi the rest oi thesystem as will be apparent later.

The control system proper consists of two directional antenna systems.One antenna system may cover a predetermined angular range in elevation,while lthe other antenna system may cover a predetermined angular rangein azimuth. The two antenna systems are located close to each other. sothat as far as any targets are concerned they may be considered to becoincident. Under normal conditions, the two fan-#shaped 'lobe patternsfrom the plane-polarized antenna systems intersect at right angles inspace with the controlled aircraft normally being disposed at theintersection of said antenna lobe patterns. 'lhe region 'of intersectionof the two beams may be controlled at will by varying the azimuth of theelevation antenna system and the elevation of the azimuth antennasystem, the two controls being inter-connected so that the patterns fromthe two antenna systems will continue to intersect. Each antenna systemhas electrical scanmeans for insuring coverage of the angular sector byeach system. Thus the elevation antenna may be an antenna producing aianshaped lobe having a comparatively small angular extent in elevation.The electrical scanning causes this lobe to move up and down so that theeective elevation coverage is extended to the desired value. The sameapplies to the azimuth antenna which, resembles the elevation systemexcept that the plane of its fan-shaped lobe is perpendicular to theplane of t'ne elevation lobe.

le each antenna might have a separate transmitter and receiver, andnally be related to each other so that the two operate as a compositesystem, it is preferred, for economy, to have certain portions of theradar system, namely transmitter and receiver, in common.- Rapidswitching of such transmitter-receiver combination between the twoantenna, i. e. azimuth and elevation antennas. is necessary.

The echoes received by the azimuth and elevation antennas appear onseparate azimuth and,

3 elevation. oscilloscopes. Actually, as illustrated in Fig. 1, eachantenna 1s provided with two separate Oscilloscopes, one being anexpanded range oscilloscope, while the other is a long rangeoscilloscope. The details of the Oscilloscopes and the manner ofpresenting the images of the echo signals may better be appreciated inconnection with the description of the system as a whole.

The ground control approach system proper' and the search system usingplan position presentation (PPI) are inter-related in that the searchsystem may select a particular range in which a target exists and makethe selected portion of the range evident on the indicators of theground control approach system proper. Thus the identity of the selectedtarget is assured when passing from search to control. The control andSearch systems are keyed synchronously and in phase so that theexploratory pulses directionally transmitted by the antenna systemsstart simultaneously.

An ordinary communication system is tied in with the control systemproper so that departures of actual glide or approach paths from desiredsafe paths may be communicated to the plane so that the plane may,either manually or automatically, correct its course in accordance withthe transmitted information.

The search system generally The plan position indicating search systemgenerally comprises an antenna I (Fig. 1) connected through a T-R box IIto a transmitter I2 and a receiver I3. Antenna I0 preferably is of thetype having a generally fan-shaped beam, the plane of the fan being in avertical plane, and, as will be explained later, is adapted to berotated in azimuth through 360. It is preferred to have the searchsystem operate on a wave length sufficiently different from the controlsystem to avoid mutual interference. The T-R box is any one of a numberof electronic switches which alternately switches the transmitter andreceiver to the antenna.

The transmitter is keyed at periodic intervals from a modulator I5 whichis common to both the search and control systems. The modulator in itssimplest form is a line pulse modulator which is controlled from asuitable timer or oscillator so that the modulator periodically suppliesa sharp, high voltage pulse to transmitter I2. The modulation in thisinstance is merely a simple change from one voltage condition at thetransmitter to another voltage condition, usually zero voltage to asudden application of full voltage to the transmitter. Any type ofmodulating system may be used for modulator I5, and examples ofmodulators may be seen in pages 533 to 545 of Radio Engineers Handbookby Terman (1943 edition), and page 291 of Ultra-High FrequencyTechniques by Brainerd (1942 edition).

The transmitter itself may be any one of a number of well-knowntransmitters operating at a desired frequency. As is well known in radarsystems, the transmitter is adapted to be pulsed for a short period oftime and be inoperative for a longer period of time during which thereceiver may receive reected echoes. 'Ihe receiver may also be one of anumber of Well known types, usually of the superheterodyne type. Thereceiver lmay be provided with a gain control I6. The receiver feeds itsoutput through a line I1 to the control grid I8 of the plan positionindicator cathode ray oscilloscope 20.

Because of more satisfactory resolution, it is preferred to useelectromagnetictypes of cathode ray tubes. However, it is understoodthat electrostatic types of tubes may be substituted therefor, inaccordance with well-known practice. Tube 20, as shown, is of theelectromagnetic type and is provided with focusing coil 22 connected toa direct current source not shown. In accordance with well-knownpractice, the intensity o1' the field due to focusing coil 22, as wellas its orientation with respect to the cathode ray tube. may be variedin order to locate the rest position of a focused beam of electrons uponthe center of the screen. In addition to focusing coil 22, the cathoderay tube is provided with a series of deflecting coils 23 forcontrolling the position of the beam on the screen. Sinceelectromagnetic types of cathode ray tubes are well known in the art, adetailed showing of the coils is unnecessary. Thus a system ofdeflecting coils for an electromagnetic type of cathode. ray tube isshown on pages 211 and 212 of the Brainerd book previously referred to.It is understood, of course, that rectangle 23 indicates a series ofdeflecting coils, usually two pairs of two coils each, and that thelines connecting these coils to the operating circuits do notnecessarily show the number of wires actually used. Thus deflectingcoils 23 are connected by lines 24 and 25 to search central system 26.Also cathode 2l and an anode 28 are connected by lines 29 and 30 tosearch central 26.

Antenna system I0, as has been previously indicated, is adapted to sweepover 360 in azimuth, and this is accomplished through a suitable drivemotor 33. In order to convey azimuth data from the antenna system tocathode ray tube 20, a sine potentiometer 34 may be driven with theantenna. This is adapted to generate two sine waves apart (a sine andcosine wave) whose instantaneous amplitudes are used for modulating twosaw-tooth waves, which will give the azimuth angle of the antenna asmeasured from some fixed reference direction to the range sweep. Suchdevices are well known in the art. (See Fig. 272 of Radar ElectronicFundamentals, War Department, December 30, 1943, Technical Manual11-466.) Sine potentiometer 34 feeds its output by cable 35 to searchcentral 26, while a trigger impulse to the search central is provided byline 36 from a suitable timer or a synchronizing oscillator for theentire system which will be described later.

Search central 26 provides sweep voltages to the deflecting coils of thecathode ray tube and a positive beam-accelerating pulse to anode 28 sothat the beam reaches the screen and becomes visible during the entiretime of sweep. It is understood, of course, that the acceleration of thebeam due to anode 28 may be controlled so that the beam is blockedexcept for the intensification due to the echo signals impressed on thecontrol grid I8 by receiver I3. However, it is usually desirable to showthe sweep trace faintly so that the instantaneous azimuth position ofthe system is visible.

If desired, faint marker traces may be impressed upon oscilloscope 20 bysuitable circuits in search central 26 connected to anode 28. Suchmarkers may show up as a series of concentric circles whose radiicorrespond to predetermined, ixed ranges. Antenna I0 produces afan-shaped beam, the plane of the fan being in a vertical plane; itconsists of radiating dipoles connected to a wave guide 40, which inturn is connected to a wave guide 4I. Wave guide 4I terminates in arotatable coupling joint 42 provided with flanges having choke sections;such joints are shown in Fig. 379, Radar Electronic Fundamentals, 'abovereferred to. To impart desirable radiation characteristics to antennasystem 4D, a suitable parabolic reiiector 43 is provided. By properdesign oi the radiators and reflector, a relatively narrow, verticallypolarized, i'anshaped beam is provided. The entire antenna system, ashas been pointed out, is rotated so that complete azimuth scan isprovided. The elevation angle of antenna l may be controlled by tiltingthe entire assembly. If desired, the antenna may be similar to theantennas used in the ground control approach system to be described indetail later and provided with a relatively iine, concentrated beampattern but variable in elevation to give the desired coverage. Othertypes of antenna systems, including parabolic reflector systems orantenna arrays with reflectors and directors, may be used as desired.'I'he search system therefore represents a plan position indicatingsystem, the pulse repetition rate of which is synchronized with thepulse repetition rate of the control system through modulator l5 whichis common to both systems..

Control system The control system comprises a synchronizer E6, (Fig. l),elevation and azimuth antennas 5| and 52, together with variouscomponents of a complete radar system and particularly including a pairof elevation indicators, a pair of azimuth indicators and an errorsystem. Synchronizer 50 includes a suitable timer 53 (Fig. 2) such as ablocking oscillator described in Fig..11 of the co-pending applicationof Haworth and Purcell, Serial No. 531,826, led April 19, 1944. Thistimer is conected, over a line 8U, to modulator l5 which, as pointed outbefore, is a line pulse modulator. A line 6| connects the output Ofmodulator l5 to a transmitter 62 of the control system. This transmittermay be any one of a number of ultra-high frequency type of transmittersutilizing magnetrons as sources of pulse energy. 'Ihe output oftransmitter 62 is connected through a wave guide and a T-R box 63 to awave guide Sti. Wave guide 66 terminates in a T section 55 leading towave guides 66 and 61, respectively. Wave guide 66 energizes elevationantenna El, while wave guide 61 energizes azimuth antenna 52.

Elevation antenna 5I scans in azimuth with Whe aid of a lever 10, whichis under manual control, as will be explained later. The elevation angleof azimuth antenna 52 may be adjusted by means of a lever 1I, as will beexplained more in detail later. Azimuth and elevation scanning is madepossible by introducing rotatable joints 12 and 13 in the respectivewave guides, these joints being of the non-radiating type, such as joint42 in the search antenna system.

Since one transmitter is common to both elevation and azimuth-f antennasystems, switching means are necessary to connect each antenna system insuccession to the transmitter-receiver combination Of the radar system.To this end, T section 65 is separated from the wave guide sections 65and S1 by narrow gaps 1s and 15. Operating within the gaps 1t and 15 area pair of metallic switch members 16 and 11, mounted on a common shaftand rotated by a suitable scanning motor 1%. Metallic members 16 and 11alternately enter the respective gaps 1i and l5 blocking communicationbetween the undesired antenna and the transmitter-receiver combination.

f' The movement of reflecting members 16 and 11 need not necessarily beuniform. Thus it may be provided that member 16, for example, which isshown in a reflecting position may be snapped Out quickly and member 11be snapped into position quickly. Thereafter the reflecting member may-be permitted to remain in position or possibly slowly turned, it beingunderstood that the reflecting member is large enough so that it fillsthe gap until a further change in switching is desired. 'I'he action ofthe antenna switching is relatively slow. Thus each antenna system maybe operatively associated with T65 for a time of the order of about 116of a second. In order to prevent detuning of the oscillator bysubstantial reection back into the transmitter, it may be desirable tohave one reflecting member entirely free of the gap before the otherreflecting member is introduced. 'I'hus some power may be wasted butoscillator stability will be maintained. Other methods for alternateswitching of the antenna systems may be utilized.

In the disclosed system a single transmitterreceiver combination is usedin alternate succession first with the azimuth antenna and then with theelevation antenna. Also, the same receiver is used first with theazimuth Oscilloscopes and then with the elevation Oscilloscopes. Sincethe output oi the receiver is connected all the time to the intensitygrids 90, 9i, 91 and 98 of all oscilloscopes over the parallel circuitsdescribed above, it becomes necessary to introduce additional circuitsfor disabling the azimuth oscilloscopes when the receiver is connectedto the elevation antennas, and for disabling the elevation Oscilloscopeswhen the receiver is connected to the azimuth antenna. Without theseadditional circuits, the elevation and azimuth antenna signals would bereproduced on the elevation as well as on the azimuth Oscilloscopes,which would render the entire system useless because Of the confusion ofthe elevation and azimuth signals on the screens of the elevation andazimuth oscilloscopes. These additional circuits comprise a keyingsystem or a gating system which blocks and unblocks first the azimuthOscilloscopes and then the elevation Oscilloscopes in alternatesuccession, and in synchronism and in phase with the alternateconnecting of the receiver first to the elevation and then to theazimuth antennas. To accomplish this proper channellzing of theelevation and azimuth echo signals to their respective Oscilloscopes, aconstant intensity light source and a motor-driven light-chopper directthe emitted light first onto one photo-electric cell channel, and thenonto the other. The outputs of these channels are used tO block andunblock the azimuth and elevation Oscilloscopes in the manner statedabove. The light chopper is driven by the same constant speed motor 18as the antenna switch 16-11, thus realizing the previously mentionedsynchronous and co-phased operation ofthe twothe antenna and theoscilloscope switching systems.

The above-mentioned oscilloscope switching system also performs anadditional useful purpose: it alternately increases and decreases theoverall gain of the receiver in synchronism and in phase with thealternate connection of this receiver to the azimuth and elevationantennas. This alternate gain control serves a very useful purpose byequalizing the amplitudes of corresponding echo signals in both theelevation and the azimuth channels in spite of the fact that theelevation and the azimuth antennas may not have equal gains, which isindeed the case. The antennas do not have equal gains because of thedifference in the precision required in the azimuth and elevation data.

To accomplish this alternating and synchronous gain control, the outputsof the two photoelectric channels are impressed respectively on twodirect current amplifiers. Since the outputs of the photo-electric cellchannels are no more than two equal-amplitude, equal duration switchingor keying signals, i. e. two rectangular waves 180 out of phase withrespectl to each other, they cannot be used directly for controlling thegain Yol' the receiver, but must undergo a suitable amplitude changecorresponding to the difference in the gain of the azimuth and elevationantennas. This is obtained by impressing one wave on one gain controlchannel and the other wave on the other gain control channel, adjustingthe outputs of the two channels to the magnitudes having an inverseratio to the ratio of the antenna gains, and controlling the gain of thereceiver in alternate succession with these signals.

The oscilloscope switching system and the receiver gain control systemsare described below.

The azimuth and elevation antennas are also connected through T-R box 63to a receiver 80 of the ground control approach system, the output ofthis receiver being connected over a line 8| to synchronizer 50. Theecho pulses, appearing in the output of the receiver, are impressed on acathode follower 82 (Fig. 2) and the output of the cathode follower isimpressed on the intensity grids of the two elevation and two azimuthOscilloscopes over a line 83, a junction point 84, a line 85, andblocking condensers 86 and 81 respectively. The left plate of condenserB6 is connected to a junction point 88 and thence through a conductor 89to the control grids 90 and 9| of the elevation Oscilloscopes 92 andCFI. As indicated in the drawing, oscilloscope 92 may indicate a rangeof approximately ten miles, while oscilloscope 93 may indicate a rangeof approximately two miles. It is understood, of course, that thesefigures are merely exemplary and, if desired, one of these tubes may beeliminated entirely. In practice, however, tube 93, with its expandedrange, enables an operator to have a more precise control of the glidingpath of the approaching planes, while tube 92, with its longer range,insures adequate presentation of a long range for early flight controlof the approaching planes.

Similarly, the right plate of condenser 81 is connected to a junction95, and this in turn is connected by wire 96 to the control grids 91 and98 of azimuth Oscilloscopes 99 and |00.

Receiver 80 is also connected to synchronizer 50 over a conductor |05.This connection controls the gain of the receiver by controlling thevoltage on some or all of the receiver amplifier tubes.

Gain control line from the receiver goes through a pair of separate,direct current ampliers |06 and |01 (Fig. 2) within synchronizer 50. Inother words, the direct current potential under the control ofamplifiers |06 and |01 determine the potential of gain control line |05so that the condition of amplifier |06 or |01 determines the gain ofreceiver 80.

It is clear that direct current amplifiers |06 and |01 cannot bothsimultaneously exercise control over the gain of receiver 80. Actually,di-

rect current amplifier I 06 controls the gain of receiver during thetime that the azimuth portion of the control system is operating, whileamplifler |01 controls the gain of the receiver during the time that theelevation portion of the system is operating. Thus the gain at thereceiver, which may be set independently for either azimuth or elevationindication purposes, is alternately controlled as elevation and azimuthswitching is effected. To accomplish this, direct current amplifiers |06and |01 are connected to lines |08 and |09, respectively, these linesentering synchronizer 50 from an elevation-azimuth commutator systemdesignated as I|0 (Fig. 1). Commutator I I0 consists of a light source II I which is adapted to shine on two photo-electric cells ||2 and U3respectively. Between the light source and the photo-electric cells,shutter discs III and ||5 may be disposed, these being on a common shaftI I6 mechanically connected so that they are driven from antenna motor18. As illustrated in Fig. 1, the two shutters IM and |I5 alternatelycut off the photo-electric cells and are timed so that the commutatingaction in switch IIO is synchronized with the antenna switching actionof the antenna switch 16-11. Thus, as shown in the drawing, azimuthantenna system 52 is operatively connected to the transmitter-receiversystem. correspondingly, shutter I|5 shuts off light from azimuthphoto-electric cell ||3. As soon as the elevation antenna system isconnected to the transmitter-receiver combination, shutter I I4 willsimultaneously be cut in between light source III and elevationphoto-electric cell II2. The outputs of photo-electric cells I|3 and II2are suitably amplified and fed to cathode followers I|1 and Illa,respectively. These cathode followers thus feed lines |09 and |08,respectively, into synchronizer 50.

Line |08, which may have suitable isolating, amplifying or cathodefollower stages within synchronizer 50 as desired, feeds line |20 comingfrom synchronizer 50 and going to junction point 95. When the azimuthindicators are to be energized, a positive potential is impressed on thecontrol grids 91 and 98 of the azimuth indicators to overcome thenormally negative potential impressed on these grids which blocks thebeams. As shown in commutator I I0, azimuth photoelectric cell I I3 hasits light source cut off so that it does not generate any potential.Thus this lack of potential may, by suitable amplifier stages. betransformed into a positive potential at control grids 91 and 98 ofazimuth indicators 90 and |00. When the azimuth indicators are cut off,by light reaching azimuth photo-electric cells |I3, the potentialgenerated by this cell will result in a negative potential impressedupon control grids 81 and 98 of the azimuth indicators to cut the beamo. The design of such photo-electric control circuits is well known andwithin the reach of anyone skilled in the art, so that detaileddescription thereof is unnecessary. It is clear that elevation-azimuthcommutator I I0 may take a different form such as ordinarypotentiometers driven by scan motor 18.

Line |09 similarly emerges, either in its identical form or as theoutput of isolating stages from synchronizer 50, as line I2| and goes tojunction 88. Thus line I2| is the analogue of line |20 but controls theelevation indicators, i. e. controls the time when the beam is on oroff.

D. C. amplifiers |06 and |01 have independent gain control circuits |22and |23, respectively, these being in the form of potentiometersaccessible to the operators. The gain controls may. be applied to onegrid of an amplifier tube, while the potential in the lead from theelevationazimuth commutator may be applied to another grid of anamplifier tube. In other words, D. C. amplifier |08 may. for example,consist of a tetrode coincidence tube followed, if desired, by furtherdirect current stages of amplification. An example of a direct currentamplifier which may be used is given on page 376 (Fig. a) of Termansbook previously referred to.

As has been previously pointed out, synchronizer 50 has within it atimer 53 i Fig. 2) for pulsing all the transmitters of the entiresystem. This timer may feed a suitable isolating stage, such as anamplifier stage |25, andthe output thereof may supply line 60 withtrigger pulses of a predetermined polarity such as, for example,positive trigger pulses. Timer 53 also may feed isolating stages,including a cathode follower |25, and provide trigger pulses of apolarity opposite to that in line 60, such as, for example, negativetrigger pulses, such pulses being applied to line 38. This line isadapted to supply trigger pulses to the various sweep circuits,including search central 2S with its sweep circuits.

Timer 53 is also the original source for range marks on the controlcathode ray tubes. Thus cathode follower |26 may feed trigger pulses toa range marker circuit |20 adapted to supply range marks for thetwo-mile sweeps. `Range marker circuits are well known in the art andmay consist of shock-excited ringing oscillators having a predeterminedfrequency with the output of the oscillator feeding suitable amplifiersand clippers to provide voltage pips at predetermined distances apart ona calibrated range sweep. Thus a ringing oscillator such as described onpage 189 of the Radar Electronic Fundamentals book may be used, togetherwith squaring, peaking and clipping circuits as described on pages 166to 186, inclusive, of the Radar Electronic Fundamentals book. Thegeneration of range marker pulses in an indicating system for a radarsystem is old and is shown, for example, in the co-pending applicationof Paul F. Brown, Serial No. 517,896, led January 11, '1944, Patent No.2,454,132.

Range marker circuit |30 may itself feed a second range marker circuitv|3| to supply markers for the ten-mile sweep ranges. This circuit maysimply consist of a count-down multivibrator system to provide, as anexample, one range marker on the ten-mile sweep for every four or ve orany other desired number of markers on the two-mile sweep. Such circuitsare well known and are described, for example, on page 219 of the RadarElectronic Fundamentals book. See also the co-pending case Whitham andHite, Serial No. 512,930, filed December 4, 1943, Patent No. 2,444,890,dated July 6, i948.

Range marker circuits |30 and |3| feed their range marker output pulsesthrough blocking condensers |33 to |36, inclusive, to four output line:numbered |40 to |43, inclusive. Lines |40 and |4| go to cathodes |45 and|46 of the two and ten-mile elevation Oscilloscopes 93 and 92,respectively. Lines |42 and |43 go to corresponding cathodes |41 and |48of the azimuth oscillovscopes. It is clear that any range markers shownon the screen of the oscilloscope as beam intensiilcation spots wouldrequire momentary, negative pulses on the cathodes. l

Means arealso provided for impressing movable identification markers onthe control and search indicators so that the same target for bothsearch and control may be identified. To this end, line |50 branches offfrom line 29 from search central 28 and goes into synchronizer 50. Line|50 may go through various ampliers, if desired, and may be fed througha cathode follower stage |5| and through blockingl condensers |52 and|53 to the ten-mile elevation and azimuth cathode leads |4| and |43. Itis, of course, possible to feed these same identification markers to thetwo-mile indicators. This, however, is hardly necessary since it isunlikely there will be more than one target on such an expanded range.Means for impressing such an identification marker are well known in theart and resemble variable range markers. Thus a circuit for generating amovable range marker is shown in the co-pending case of Garman andStafford, Serial No. 522,937, filed February 18, 1944, Patent No.2,573,070 granted October 30, 1951; and while this circuit is shown asapplied to an electrostatic type of cathode ray tube, its application toan electromagnetic type having a polar type of presentation is Wellknown in the art. It is understood that suitable manual means under thecontrol of the search operator may be provided for generating a markerin proximity to a selected target echo or even a pair of markers on eachside of a target echo, the target echo being bracketed on the range axisbetween such markers. Such marker pulses of negative polarity may beapplied to the-cathodes of the indica' tors.

There is also provided a means for generating a marker line on thecontrol indicator for showing a predetermined direction line on theazimuth oscilloscope and a predetermined elevation line, such as thelground line, on the elevation oscilloscope. Thus, for example, as willbe explained later, it is 'desirable to present on the screen as anintense trace a line showing the location of the ground on the elevationOscilloscopes. and some line having a fixed bearing with'reference tothe runway on the azimuth oscilloscopes. To provide this information, apair of leads |56 and |51, fed from suitable means to be laterdescribed, provide voltages whose polarity and magnitude are a functionof the instantaneous position of the control antenna beams as controlledby the scanning motor 18. In other Words, leads |55 and |5'| giveindications of the instantaneous beam elevation of the elevation antenna5| and beam azimuth of the azimuth antenna 52. This information may befed either directly or indirectly through suitable means to one-kickmultivibrators |59 and |50. Such multivibrators are well known in theart, and an example of one is given in Fig. 226 of Radar ElectronicFundamentals.

Since the scanning action due to motor 18 is relatively slow, the beamfrom each antenna systern may dwell on a desired point for a substantialperiod of time as compared to the duration of each pulse of highfrequency energy emitted by the transmitter. Thus, for example, if thepulse repetition frequency of the. systemis about 1500 per second, andthe scanning movement is something of the order of one-half to onecomplete scan per second, then it is possible for each of muitivibrators|59 and |60 to remain in their 11 unstable position for a period ofabout V of a second before kicking back to their normal position.

If the elevation and azimuth antennas are switched back and forth at therate of about ten times per second. it will be possible to show up theground and other lilies on the Oscilloscopes irrespective of the phaserelation between the scanning movements of the two antennas and theswitching between the azimuth and elevation antennas. It is understood.of course, that any other time may be chosen, and since the cathode raytubes preferably have long persistence screens. it may be possible tocut the time of multivibrators |59 and |60 down to between 1/5 of asecond and ,la of a second or even lower. Each of the multivibrators isequipped with variable control for controlling the voltage at which themultivibrator will kick off. Hence it is possible to adjust eachmultivibrator so that it will kick off when the corresponding antennasystem is at a desired setting.

Multivibrator |59 feeds its output through blocking condensers |6| and|62 to the elevation oscilloscope lines |40 and |4|, while multivibrator|60 feeds its output through blocking condensers |63 and |64 to theazimuth oscilloscope lines |42 and |43.

It is understood that the circuit for feeding the outputs of the rangemarkers, identification marker and multivibrator pulses to the variouscathode leads is merely diagrammatic. Actually, as is evident from thedrawing, it will be necessary to put in some isolating stages betweensome or all of the various blocking condensers and the linesto theindicators to prevent undesired feed into other lines. Thus, as anexample, isolating stages such as cathode followers providing one-waytransmission only may be provided between blocking condensers 6| and|62, as one unit, between blocking condensers |63 and |64 as anotherunit and between blocking condensers |52 and |53 as a third unit. Inthis way a range marker pulse from circuit |30, for example, may passthrough blocking condenser |33 to line |40 and will be prevented fromgoing beyond f blocking condenser |6|. Similarly, marker pulses throughblocking condenser |34 would go to line |42 and would be prevented fromdigressing through blocking condenser |63. Inasmuch as such isolatingone-wayI valve systems are old in i the art, no definite showing isdeemed necessary. The various arrowheads shown in Fig. 2 are consideredto include suitable one-way transmitting circuits.

In Figs. 3 and 4, line 36 supplies trigger pulses originating in timer53 to elevation sweep amplier system |10 and azimuth sweep ampliersystem |1|. Referring first to system |10 (Fig. 3), there are providedtwo sweep systems for the ten-mile and two-mile Oscilloscopes,respectively. Line 36 connects through lead |13 to one-kickmultivibrators |14 and |15. A multivibrator of this type is shown inFig. 228 of the Radar Electronic Fundamentals book. Thus multivibrator|14 is adjusted so that it generates a gate of about 22 microseconds forthe two-mile oscilloscope range. Similarly, multivibrator provides agate of about 4111 microseconds for the ten-mile range. It is understoodthat the gate duration is determined by the range. With electromagneticOscilloscopes, it may also be necessary to provide some additional timeto overcome initial self-inductance. Thus starting the oscilloscope coilcurrent a short time ahead of the energy from the antenna when magneticcon- Al2 4 trol is used is,well known and may be provided in variousways.

Multivibrator 14 may feed a positive gate to .cathode follower |16connected to oscilloscope anode |11a to generate a beam. Multivibrator|14 also contrpls sweep generating circuits |11 and |18 whose'outputsmay be fed through cathode followers |19 and |80 to the defiecting coilsof the two-mile oscilloscope. The two-sweep generators are necessary toprovide the two components for controlling the beam direction. Theactual amplitude of sweeps must be controlled in accordance with theelevation of the antenna pattern. To this end, elevation line |56 isconnected in to sweep generator systems |11 and |18. Theconnection togenerator |10 may be through a phase inverter i8 Sweep generators v|11and |18 as controlled by elevation angle potentials may resemble thevariable amplitude sweep generators described on page 58 et seq. olRadar System Fundamentals. It will be understood by those skilled in theart that the indicators 92, 93, 99, and |00, as well as search indicator20, include suitable power supply circuits for energizing the cathoderay tubes and focusing, intensifying, and controlling the initialposition of the electron beams. Since such circuits are complex and veryWell known, they are, for the sake of simplicity. not illustrated.Examples of common circuits of this type are shown in Fig. 303, page49'7, of Principles of Television Engineering by Donald G. Fink, and onpages 553 and 556 of Television by Zworykin and Morton. The idea ofgenerating two sweeps whose amplitudes vary sinusoidally so that rotarymovement of radial beam sweeps will result 1s well known. The systems ofthis type, when applied to radar, are known as plan position indicatingsystems, or PPI. It is also possible to mechanically turn the magneticdeflecting coils and have a simple sweep generator of constant amplitudewhich also produces the PPI presentations. In the Oscilloscopes of thecontrol system, the actual angles from the antenna systems aremultiplied in a non-linear or eccentric manner and the point of originof the sweep is displaced from its normal control position to the onewhich is along the periphery of the screen, as illustrated in Figs. 1,l1 and 12. It is evident that the steady space current of cathodefollowers |19, flowing through the vertical and horizontal deflectioncoils will displace the origin of the sweeps toward the periphery of thescreen of tube 93, and the origin of the sweeps of the other indicatorswill be displaced similarly. Of course, other conventional means such asthe beam positioning controls shown in Fink and in Zworykin and Morton,supra, may be utilized to position the electron beams as indicated inFigs. 1, 11 and 12. Thus the movement of the range sweep is shown ascovering an angle of about 60 whereas the actual antenna beam movementmay -be l/g or 1/4 of that. The mechanical movement of the oscilloscopedeecting coils could be multiplied through gearing providing the driveis of eccentric type so that the amplitude is non-linear. The motor ofthe corresponding antenna system Would naturally be used to control thedelecting coil movements.

Because of the rapidity of the sweeps in the two-mile range, clampingtubes may be necessary to stabilize voltages at the ends of the sweepsin preparation for a new cycle of operation. Such expedients are wellknown in the art.

Multivibrator |15 controls the ten-mile osciland the synchronization ofthis control with the angular positions of the azimuth and elevationantennas is described later in the specification.

Control antenna system The antennas used in the control portion of thesystem may take on a variety of forms. Thus it is possible to providesubstantially point sources such as one or more dipoles at the focusofva paraboloid for each of systems and 52. For elevation antenna 5| itwould then be necessary to move the paraboloid through a limitedvertical angle so that the beam from the paraboloid may cover apredetermined elevation range, such as about as an example. In additionthereto, an operator control through arm 10 would adjust the azimuth ofthe antenna. Similarly, azimuth antenna 52 might be moved over anazimuth range by a suitable motor, while arm 7| would be used to controlthe elevation.

It is preferred, however, to use an antenna system consisting of anarray of dipoles set in a variable width wave guide. Such an antennasystem is disclosed and claimed in the copending application of Luis W.Alvarez, Serial No. 509,790, filed November 10, 1943. In this system itis understood that the dipoles may be replaced by open apertures throughwhich energy may issue. Also several parallel series of dipoles may beprovided.

The preferred antenna system has desirable radiation characteristics,and a brief description of the system is herewith given in connectionwith Figs. 5 to 9, inclusive. Referring, therefore, to these figures, awave guide 200 having a variable width is provided. Wave guide 200 mayconsist of a generally U-shaped base member 20| having side portions 202and 203, respectively. Thisbase member may be formed of any suitablematerial such as brass, aluminum or even steel. If made of steel, it ispreferred to have the active wave guide surfaces plated with a goodconducting metal such as copper or silver. Portion 20| of the wave guidemay have an upstanding flange 206 which functions as a reector and isdisposed at a predetermined distance behind an array of dipoles 206, asshown.

Disposed within wave guide 200 isxa generally L-shaped movable structure2|0 having an upstanding portion 2li forming one of the narrowdimensions of the movable wave guide. Base member 20| has acorresponding flange 2|2 cooperating with flange 2|| to determine theremaining narrow guide dimension. Each of these flanges 2li and 2|2 areprovided with slots 2|3 throughout their length. Movable member 2|0 ismaintained at a fixed, spaced relationship with member 20| by anysuitable means such as a pair of rollers 2|5, so that small gaps betweenflanges 2| and 2| 2 on the one hand and the long wave guide sidesdetermined by members 20| and 2|0 on the other hand are formed. As shownin the drawing, the gaps are exaggerated, and in practice they would besomething of the order of several thousandths of an inch.

The combination of gap plus slot provides a substantially half-wavelength path from the interior of the wave guide to the bottom of eachslot 2|3. Thus a choking action is created tending to confine theradiant energy to the interior of the wave guide. Such chokes aredescribed and claimed in the co-pending application of Salisbury, SerialNo. 489,844, filed June 5, 1943, Patent No. 2,451,876, dated October 19,1948.

It is clear that L-shaped member 2I0 may be moved perpendicular to thelong dimension of the wave guide, i. e. horizontally as seen in Fig. 9.Means for moving member 2|0 may consist of a toggle arrangementcomprising toggle arms 220 and 22| pivoted respectively on the fixed andmovable portion of the wave guide system. A draw bar 222 may bepivotally mounted to open or close the toggle as the case may be andthus control the width of the wave guide. It is understood that inpractice the wave guide system may be something of the order of between5 and 10 feet long with the toggle operating means repeated at suitableintervals. In order to maintain movable member 2|0 in proper position, aspring 225 carrying a roller 226 may be provided at suitable intervals,the roller bearing on member 2|0 so that the system is maintained inposition. Scan motor 'I8 is adapted to drive draw bar 222 through asuitable cam, it being understood that the drive is so arranged that thevariation of guide width follows a predetermnied pattern depending uponthe characteristics of the entire system.

Carried in main guide member 200 are a series of dipoles 228 (Fig. 7').AThese dipoles may be formed in any suitable manner and, as shown here,may consist of a hollow, cylindrical body 229, the upper portion ofwhich may have two diametral slots 230. At right angles to the diameterjoining the slots are fingers 23| and 232 forming the dipole radiatingelements proper. Finger 23| is merely fastened to the outside of body229 at the upper end thereof, while finger 232 actually goes throughthis body and may be fastened thereto if desired and continues to acentral rod member 233 extending down through body 229 and into the waveguide region. 'I'he proportion of the various elements including slotsand fingers depends upon the Wave length used, it being understood thatslots 230 function as chokes so that fingers 23| and 232 may beoppositely poled. The general construction of the dipoles may be asshown or they may be constructed as disclosed in the co-pendingapplication of R. W. Wright, Serial No. 511,868, filed November 26,1943, or the application of L. G. Van Atta, Serial No. 507,585, filedOctober 25, 1943, Patent No. 2,486,620. I

As clearly indicated in Fig. 7, adjacent dipoles are mounted so as to beeffectively spaced about one-half wave length apart, the exact one-halfwave length spacing being present when the beam from the antenna arrayis directed substantially straight ahead. It is clear from the drawingthat adjacent dipoles are reversed so that in spite of the one-half Wavelength separation, the feed for adjacent dipoles is in phase.

A large reflector 240 may be provided at a suitable distance from thearray of dipoles 228 and ange 205 so that the entire antenna system hashighly directional characteristics. It is understood that as thedimension of the wave guide varies, the effective spacing betweenadjacent dipoles departs from one-half wave length and thus results inthe beam being bentI away from the direction normal tothe plane of thearray. The size of large reflector 240 and its position are such that itreects energy emitted from the dipole array under all conditions.

Energy from the transmitter may be fed in at one end of the wave guidesystem, and in order to prevent undesired reflection back to thetransmitter, a suitable absorbing means at the remote end of the waveguide system may be provided. Such absorbing means is shown in Fig. 6and may consist of a wave guide section 24| carrying within it a mixture242 of suitable absorbing material such as sand and aquadag maintainedin place by some insulating sheet 243. It is preferred to haveinsulating sheet 243 inclined at an angle and retain the absorbingmedium in this position so that reflection from the absorbing load maybe eliminated. Cooling fins 245 may be provided to dissipate heat. Theentire absorber may be suitably mountedupon the end of the wave guidesystem by means of a choke flange coupling 246.

At the input end of the wave guide system. it is necessary to provide atransition section for joining the variable wave guide portion with thefixed wave guide of the system. To this end, a plate member 256 (Fig. 8)pivoted at 25| may be provided separate from but generally as acontinuation of movable guide member 2|0. Plate member.250 is providedwith flange 252 with choking slot 253 generally in line with thecorresponding flange and slot in movable guide member 2|0. A leverarrangement 255 connects movable member 2|0 with plate 25|] so that thelatter tends to pivot on pin 25|. It is clear that the wave guide at theend of the system, namely adjacent pin 25|, has little variation. indimension so that a fixed wave guide 251 may be fastened thereto. Fixedwave guide 251 goes down to rotary joint 12 or 13 as the case may be.

As has been previously pointed out, it is necessary to provide anindication at both the synchronizer and the sweep amplifier systems ofthe position of the beam or pattern from the antenna system due to thevariation of Wave guide dimension. While a simple potentiometerarrangement might be provided, the potentiometer being driven from motor18 which drives movable guide member 2|0, it is preferred to provide thearrangement disclosed herein. To this end, a variable condenser 260 isconnected in parallel to a fixed grounded condenser 26|. In series withcondenser 26| is another condenser 262. A radio frequency oscillator 263supplies radio frequency voltage to condenser 262 and parallelcondensers 260 and 26|. A rectifier 264 is connected across parallelcondensers 266 and 26| and feeds the instantaneous, rectified potentialto a cathode follower 265 and thence to line |56. This varying directcurrent voltage, representing in electrical terms, the angular positionof the elevation lobe with respect to the horizontal line. is impressedon the cathode follower 265, and the output of the latter is impressedover conductor |56 on the sweep circuits |11|18 and |11a|18a, furnishingthe necessary sweep voltages for the stationary deflection coils of theZ-mile and 1 0-mile elevation Oscilloscopes, where it varies theamplitudes of the sweep voltage waves to produce the types ofpresentations illustrated in Figs. 1, l1 and 12. The same system withprimes is provided for the azimuth antenna.

Variable condenser 260 is mechanically coupled to an oscillating arm 222connected to and oscillated by the elevation antenna and the plates ofthis condenser are so cut that there is a predetermined variation ofcapacitance with the angular rotation of the elevation antenna. Byproperly proportioning the various condensers and their characteristics,it is possible to obtain in line |56 or line |51, as the case may be, arectified voltage whose amplitude and polarity, although the latter isnot essential, have any desired relationship to the position of themovable guide member. In the disclosed system the condenser is shaped toimpress a radio frequency voltage to produce a display on theoscilloscope screens having uniform magnification (approximately 3 to 10times) for all distances measured in a specified direction no matterwhat part of the display pattern is used. The specified direction inthis case is the direction substantially at right angles to lines 363'and 369 illustrated in Figs. 11 and 12.

With this type of magnification, if the X-axis is made to coincide withthe line 363 or 369, the magnification will be along the Y-axis. Sincethe cursor arms are moved along the Y-axis. and the magnification alongthe Y-axis is made uniform for all distances (i. e. range distances) nomatter what part of the range is used, the response of the errorindicators is uniform over the entire range. To illustrate: a distanceof one millimeter along the Y-axis as measured on the oscilloscopescreen will correspond to, say, 5 feet distance in space along theazimuth or elevation lobe irrespective of range. Because of this type ofmagnification the error meters and the potentiometers have uniformgraduations.

It has been pointed out that elevation antenna system 5| may be adjustedin azimuth, and conversely azimuth antenna system 52 may be adjusted inelevation. In order for the entire system as a whole to operate, it isessential that the radiation patterns from these two antenna systemsintersect in space at the target.

In order t0 obtain this relationship, elevation antenna system l5| hasan arm 10 which is mechanically connected through a suitable linkage andchain system to a pair of foot pedals 210 at the azimuth indicators, the.mechanical con'- nections shown by dotted lines. Since the azimuthindicators are the only indicators in the whole system which giveazimuth, it is necessary that the azimuth position of antenna system 5|be under the control of the azimuth Operator. This operator is naturallyat the azimuth indicator. Foot pedals 210 are also mechanically tied toa cursor 21| suitably pivoted so as to sweep over the outside face ofthe two-mile oscilloscope screen.

Similarly. a cursor 212 on the ten-mile scope screen is provided. Thesecursors are simply strips of material, preferably translucent, such asCelluloid or Lucite, and may have engraved thereon two lines, as shown,forming a. small angle. By suitable calibration, it is possible to havecursors 21| and 212 aligned with a target echo being received by thesystem. This alignment, of course, depends upon the alignment ofelevation and azimuth antenna systems and initially would require somefixed echo upon which the entire system could be trained. By suitablemechanical linkage, cursors 21| and 212 may be moved in proper relationto elevation antenna system 5| so that antenna system 5| is aimed at atarget indicated as being within the angle marks on the two cursors 21|and 212. Thus as a desired target echo varies in azimuth, assuming it iswithin the range of azimuth system 52, the operator at the azimuthindicators can keep the elevation antenna system 5| properly trained.

Conversely, the operator at the elevation indicaters is provided withfoot pedals 215 mechan- 17 ically linked to azimuth antenna 52 andcontrolling the elevation thereof. These foot pedals are also connectedto control elevation cursors 216 and 211.

Error indicating system It is desirable to provide means for indicatingthe departure of the actual glide path or approach path taken by a planeunder control of the system from a prescribed or desirable glide path.To this end, means are provided for indicating departure from azimuth ofthe actual path as compared to the desired path, and means are alsoprovided for indicating a dangerous elevation. To provide azimuth errorindications, the azimuth operator may have an error control hand wheel288 mechanically linked to error cursors 28| and 282 on the azimuthindicators and an error potentiometer 283. Like the antenna ccntrolcursors, error cursors 28| and 282 are merely strips of flexible,translucent material suitably pivoted and may have a guiding line etchedor inscribed down the length of the cursor. The selected plane targetappearing on the azimuth indicators is followed by moving the errorcursors so that the center thereof passes through the target echo. It isunderstood, of course, that the antenna cursors 21| and 212 aresimilarly controlled so that the selected target echo is correctlydisposed with relation thereto.

Upon operation of error hand wheel 288, potentiometer 283 is varied, andthis in turn controls an aural unit 285. This aural unit contains meanswhereby a note of increasing pitch is generated as the differencebetween the plane azimuth and the desired azimuth increases. In order todistinguish positive or negative azimuth differences, the note may bebroken up to consist of a series of dots or dashes. The means foraccomplishing this may take on a variety of forms. As shown in Fig. 10,potentiometer 283 may have one line 286 going to control grids 281 and288 of a pair of vacuum tubes 289 and 298. Vacuum tube 289 has its anode29| connected to a suitable source of B+ potential, while its cathode292 may be connected through a junction point 293 to the high side of aload resistor 298 and thence to ground. It is understood, of course,that the other terminal of control potentiometer 283 is grounded tocomplete the circuit, and that a suitable source of potential isincluded.

Vacuum ltube 288 has its cathode 298 grounded, while anode 298 may beconnected through a load resistor 388 to a suitable source of B+potential. Anode 289 may be directly connected to control grid 38| of aD. C. coupled amplifier tube 382. Tube 382 has its anode 383 connectedto a suitable source of B+ potential, while its cathode 388 is connectedto the high side of load resistor 288. Junction 283 may be connected toa relaxation type of oscillator 386.

A simple form of relaxation oscillator useful in this system is shown inFig. 35d, of page 515 of Termans Radio Engineers Handbook. In anoscillator of this type, as the potential of junction 283 rises, thefrequency of the oscillator rises since the control grid of the gas tubereaches the firing point more quickly. Other types of oscillators may beused. A gas tube type of oscillator is only responsive to voltageincreases of one polarity, say positive if impressed upon the grid.Hence the system of tubes 289, 298 and 382 converts variation ofpotentiometer voltages in 283 into positive voltage changes.

In order to take account of the sign of the error azimuth angle, theline to control grid 283 is extended to control grid 3|8 of a vacuumtube 3|| having a cathode 3|2 suitably grounded. Tube 3|| may have itsanode 3| 3 connected through a multivibrator 3I5 to a suitable source ofB+ potential. Thus tube 3I| is adapted to function as a switch inresponse to positive voltages and complete the multivibrator energizingcircuit. Multivibrator 3I5 preferably operates at a slow rate such asone cycle per second. The output of multivibrator 3|5 may be fed througha blocking condenser 3|6 and combined with the output of relaxationoscillator 306 to junction point 381. The combined output is fed intocontrol grid 388 of a cathode follower tube 388a whose anode 389 may beconnected to a suitable source of B+ potential. Cathode follower tube388a has its cathode 389a connected through a suitable load resistor399b to ground, and the output at the cathode may be taken along line3|6.

Line 3|6 may go to the modulating portion of the transmitting part of acommunication system 3|8 which may have communication with incomingplanes. It is clear that as potentiometer 283 is varied the aural signalunit may -be controlled as above indicated to communicate to the planean indication of the azimuth thereof. If the plane is coming in at thecorrect azimuth for the path portion of the approach, potentiometer 283will be at an intermediate position so that the output of line 3 I 6 maybe a series of dots, each dot being a low-pitched sound. If the azimuthof the plane departs from the desired value. the pitch will increase. Onone side of the desired azimuth, the pitch will be continuous, whereason the other side the pitched note will be broken up into a series ofdots. If desired, means may be provided such as by a blocking condenserto increase the intensity of the output in line 3|6 with increase inpitch frequency.

At the elevation indicators, the operator is provided with an error handwheel 328 mechanically connected to error cursors 32| and 322 on theelevation Oscilloscopes and also connected to an elevation errorpotentiometer 323. It is understood that the elevation operator wil1operate hand wheel 328 to keep the cursors aligned with the desiredplane target and in doing so will Vary the position of potentiometer323. Potentiometer 323 is connected by a line 328 to aural unit 285.Line 324 may go to control grid 325 of a gas-filled triode 326 whoseanode 321 is connected to ground through an alternating source ofpotential 328. Gas triode 328 has its cathode 338 connected by line 33|to a warning light 333 in a visual error indicating panel 334 (Fig. 1).It is understood, of course. that potentiometer 323 is connected throughto cathode 338 of the gas triode to complete the circuit, and theshowing here is diagrammatic. By properly setting potentiometer 323, anydesired elevation glide path may be chosen to trip warning light 333.Thus if a glide path below a minimum elevation is set as the minimumpath, then potentiometer 323 may be so poled that a positive potentialis impressed upon control grid 325 of gas triode 326 to cause breakdown.As long as the grid remains above firing potential, lamp 333 will glow.As soon as potentiometer 323 is changed to reduce grid 325 below itsfiring potential, then alternating supply 328 will permit tube 326 toagain resume control.

While no automatic means for communicating to the plane are shown in theevent that warning light 333 glows, it is understood that such may beprovided. Thus the break-down of gas tube 326 may set into operation apredetermined warning signal in the aural signal unit for transmissionthrough the communication system 3|3. However, in order to avoidconfusion with the azimuth signal system, a suitable telephone at theelevation operators position may be provided so that the elevationoperator may orally warn the pilot of the incoming plane.

In the event that the signal to be transmitted Afrom the communicationsystem 3|8 to the incoming plane is to control automatic apparatus inthe plane rather than put the pilot on notice, then it is clear thatmeans may be provided under the control of elevation error potentiometer323 for sending out proper signals. Thus it may be possible forelevation error potentiometer 323 to control a system broadly similar toazimuth aural system by operating in an entirely different frequencyrange. Thus, as an example, the azimuth error portion of the systemmight operate over an audio-frequency range from about 25 cycles persecond to 250 cycles per second. The elevation portion of the systemmight operate from about 1500 cycles per second to about 3,000 cyclesper second. Commutating means may be provided to alternately switch theazimuth and elevation portions of the system to communication system3|8. Thus the commutator could operate to switch the azimuth portion ofthe aural unit to communication system 3|8 for three seconds and thenconnect the elevation portion of the aural system for succeeding threeseconds. Other combinations are possible.

In order that an operator may have a visual indication of errordifference between the actual and desired glide path, meters 350 and 35|may be connected to lines 286 and 324, respectively, so as to indicateazimuth and elevation error. These meters are merely voltmeters toindicate the position of potentiometers 283 and 323 and may becalibrated in any manner desired. Thus azimuth meter 350 may becalibrated in terms of angle, while elevation meter 35| may also becalibrated in terms of angle. It is thus possible ior an operator or atrailc director to have the error panel in front of him containing thetwo error meters and warning lights, and communicate verbal directionsto an incoming plane. Thus the complexity of the aural system may -beeliminated, ii' desired.

' Indicator markings Figs. 11 and 12 show markings as they might appearon the various control indicator tubes. The search indicatoroscilloscope (not shown) will merely shown conventional plan positionindications with range markings, namely radial sweeps which may or maynot be visible by themselves but which will show up targets as brightspots. As the search antenna system rotates in azimuth, the sweep withinthe indicator oscilloscope will also rotate.

Referring to Figs. 11A and B, the drawing shows the face of the ten-mileelevation and azimuth Oscilloscopes, respectively. In each case, points360 represent the start oi' the sweeps and thus indicate the location ofthe control equipment. Lines 36| and 362 represent respectively thenegative and positive angles of elevation scanned by the elevationantenna, while 363 represents the horizontal ground line. Echo 363 mayrepresent a hill or similar target, while the other spots therein mayindicate various targets. Thus spots near the maximum elevationlboundary 362 may repreesnt planes, while spots near the horizontal line363 may represent trees or the like.

Referring now to Fig. 11B. lines 361 and 333 represent the extremeboundaries of the azimuth angle scanned by the azimuth antenna, whileline 369 may represent a line of predetermined azimuth such as a lineparallel to runway 313. Target echo 364 is the same as in the precedinggure and shows the azimuth of the hill. Remaining targets may showincoming planes. The series o! equi-distant, parallel lines may indicaterange markers. Thus with the ten-mile range, these markers may be aboutone mile apart. An identification marker line 31| in both oscilloscopesmay be placed thereby the search operator to indicate a plane takenunder control from the search system.

Referring now to parts C and D of Fig. il, the numbers there indicatethe corresponding sweep lines as in Figs. A and B, it being understood,however, that C and D show the two-mile oscilloscopes. In order to get abearing in azimuth, an articial target such as a suitable reflector maybe disposed so that the target echo shows as spot 316 in Fig. D. Thusthe operating system may be calibrated for azimuth, this beingparticularly important of the two-miie oscilloscope. The alignment ofthe ten-mile azimuth oscilloscope need not be so precise.

Figs. 12A and B are simplined showings of the azimuth oscilloscopesillustrating the azimuth alignment, the numbering there corresponding tothat in Figs. 11B and 11D, respectively.

During the operation of the entire system, an incoming plane may nrst bepicked up by the search system. When this plane is at a predeterminedazimuth, elevation and range, it may be passed on to the control system,the plane being identified by the range marker generated in the searchsystem and shown on the search and control indicators. Once the controlsystem has taken control of a plane, the azimuth, elevation and rangemay be accurately indicated and directions may be given to the plane.'I'hus at the azimuth station, the foot pedal for controlling theazimuth of the elevation system is manipulated in such a manner that thecorresponding cursor on the azimuth tube is properly positioned aboutthe target echo. Similarly, the error cursor is manipulated by the handwheel. 'I'he same procedure at the other indicator tube is carried out.

It is clear that the intersection oi' the error cursor and ground lineat the elevation tube will give the landing point and also indicate theglide angle. Similarly. the intersection of the error cursor and azimuthmarker line parallel to the runway may determine an azimuth limit. Thusthe indicators furnish instantaneous, visible data showing the positionof the plane and also whether or not the glide path of the plane issuitable. A separate operating position at the control tower in a tramecontrol station i'or an airfield may have the error panel with the errormeters and warning light.

What is claimed is:

1. A radar system having elevation and azimuth radar channels havingdirectional elevation and azimuth antennas respectively, means forimpressing exploratory pulses on said antennas, elevation and azimuthcathode ray tubes each tube having a screen connected to the respectiveantennas, and for indicating target echoes, means for oscillating eachantenna through a limited sector. the sector of the elevation antennabeing in elevation, and the sector of the azimuth antenna being in'azimuth, first means at the elevation tube for controlling the elevationof said azimuth antenna, and second means at the azimuth tube forcontrolling the azimuth of said elevationantenna whereby said twoscanning sectors may be caused to intersect on a common target.

2. The system as defined in claim 1 wherein each tube is provided with amanually movable cursor on the face thereof, said cursor being manuallymoved to follow a selected target indication, and means for connectingsaid cursor and the respective antenna for moving said antenna to directits radiation pattern on the selected target.

3. The system as defined in claim 1 which also includes a radiocommunication channel an error cursor movably mounted on the face ofeach tube, said error cursor following the selected target indication onthe respective screen, first and second means controlled by the movementof the respective cursor, said first and second means generatingrespective error signals corresponding to the position of said errorcursor, and connections between said communication channel and saidfirst and second means for controlling the intelligence signaltransmitted by said communication channel.

4. The system as defined in claim 1 wherein at least one indicating tubeis provided with an error cursor movably mounted on the face thereof,manually operated means for making said error cursor to follow a targetindication, means controlled by said manually operated means forgenerating error signals, and an error indicator connected to said lastmeans for visually indicating the departure of the actual path of theselected target from a predetermined path.

5. A radar system having elevation and azimuth channels havingrespectively elevation and azimuth directional antennas, elevation andazimuth cathode ray tubes each having a. screen, means for producing asector of plan position indication of target echoes on each screen. andadditional means yfor maintaining the center of said sector off thecenter of said screen, means at each antenna for scanning a limitedsector in azimuth with said azimuth antenna, and a limited elevationsector with said elevation antenna, means at the elevation tube forcontrolling the elevation of said azimuth antenna, and means at theazimuth tube for controlling the azimuth of said elevation antennawhereby said two scanning sectors may be caused to intersect on a commontarget.

6. The system as defined in claim 1 in which said azimuth channelincludes a full range azimuth tube and a vernier range azimuth tube, andsaid elevation channel includes full range and Vernier range elevationtubes.

'7. A radar system as defined in claim 1 in which said first and secondmeans are two cursors movable over the faces of the elevation andazimuth tubes respectively to follow the image of the selected target onsaid screens, and instrumentalities operated by said cursors, forpointing said azimuth and elevation antennas at said selected target.

8. A radar system including a directional antenna, means forperiodically impressing an exploratory pulse on said antenna, meanscausing said antenna to scan a limited angle, a cathoderay tube having ascreen, a sweep generator initiating a sweep in timed relation to saidpulse, defiecting means 'for said tube, said deflecting means beingconnected to said generator for producing a polar indication in terms ofsaid angle and range of objects producing echoes of said pulse, meansconnected to said tube to locate the polar origin of said sweepssubstantially away from the center of said screen, circuits within saidsweep generator for angularly expanding said polar indication, and anantenna angle synchronizing means interconnecting said antenna and saidsweep generator, whereby the polar presentations on said screen have apredetermined angular relation to the angular position of said antenna.

9. A radar system as defined in claim 8 in which said circuits withinsaid sweep generator have amplifying means for angularly expanding saidpolar indication, said amplifying means having parameters to produce asubstantially uniform magnification measured in a direction atsubstantially right angles to a predetermined radius of said polarpresentation Ifor all ranges and angular positions of said antenna.

10. A radar system including a directional antenna, scanning means foroscillating said antenna, a cathode-ray tube having a screen,beamdeflecting means and means for generating the 'cathode-rayvbeam;means connected to said tube for normally positioning said beamsubstantially at the periphery of said screen, first and secondsweep-generating means connected to said beamdeflecting means, saidsweep means generating, respectively, first and second sweep voltages;and a source of varying direct voltage connected to said firstsweep-generating means for controlling the amplitude of said first sweepvoltage as a function of angular position of said antenna, said firstsweep-generating means being responsive to said varying voltage toproduce a substantially uniform uni-lateral magnification of allpresentations on said screen.

11. A radar system as defined in claim 10 which also includes amechanical coupling between said antenna and said source of varyingdirect voltage for making the amplitude of said voltage substantiallyproportionate to the angular position of said antenna with respect to apredetermined reference line for the oscillations of said antenna.

12. A radar system including a directional antenna; scanning means foroscillating said antenna; a cathode-ray tube having a screen, means forgenerating a cathode-ray beam, and beam-defiecting means; first andsecond sweepgenerating means connected to said beamdefiecting means,said sweep means generating, respectively, first and second sweepvoltages; a source of varying direct voltage whose amplitude is afunction of angular position of said antenna with respect to apredetermined reference line for the oscillations of said antenna; andfirst and second connections between said source and said first andsecond sweep-generating means, respectively, for controlling theamplitudes of said voltages as first and second functions, respectively,of the angular position of said antenna, said first function havingmaximum' limits greater than the maximum limit of said second function.

13. In a radio system, a cathode-ray oscilloscope including acathode-ray tube having a screen, means for generating a cathode-raybeam, and first and second beam-deflecting means, first and secondsweep-generating means connected respectively to said first and secondbeam-deflecting means, said sweep means generating, respectively, rstand second sweep voltages, a source oi' varying direct potential, andilrst and second connections between said source and said tlrst andsecond sweep-generating means respectively, said rst connectionincluding means to vary the amplitude of said rst sweep voltage betweenzero amplitude and a predetermined maximum amplitude of one polarityonly which is greater than the maximum amplitude of said second sweepvoltage, and said second connection including means to vary theamplitude of said second sweep voltage between a maximum amplitude whichis smaller than the maximum amplitude of said iirst voltage. and aminimum amplitude which is greater than zero. y

14. In a radio system, a cathode-ray oscilloscope including acathode-ray tube having a screen, means for generating a cathode-raybeam, and beam-deecting means for deecting said beam along twocoordinates; said beam-deilecting means normally positioning said beamalongthe periphery of said screen; first and second sweepgeneratingmeans connected to said beam-deflecting means and generating,respectively, rst and second sawtooth sweep voltages; a source of varying direct voltage connected to said rst sweepgenerating means, andmeans for successively varying the amplitude of said rst sweep voltagein accordance with said varying direct voltage, while said second sweepvoltage remains substantially constant.

15. A ground control approach system comprising a transmitter, elevationand azimuth antennas, rst means for alternately connecting said antennasto said transmitter, a receiver, said flrst means alternately connectingsaid receiver to said antennas, azimuth-range and elevation-rangeantennas at said object, said means being also' connected to andoperating said arms, whereby the positions of the arms determine thepositions of said antennas.

16. A ground control approach system as denned in claim 15 which alsoincludes a second set of arms on the faces of said Oscilloscopes tofollow the positions of said image, and a com- 50 munication systemtransmitting intelligence signais to said object. said intelligencesignals being controlled by said second set of arms.

17. A plan position indicating radar system including a receiver, acathode ray tube connected tosaid receiver and having beam deilectingmeans and a fluorescent screen, means for the presentation of signalsimpressed on said tube by said receiver along polar coordinates on saidscreen. means coupled to said deflecting means for locating the polarorigin of said presentation substantially at the periphery of saidscreen, and further means coupled to said deiiecting means for expandingsaid polar coordinate presentation in the angular direction.

18. A plan position indicating radar system including a receiver, acathode ray Ytube connected to said receiver and having beam deilectingmeans and a fluorescent screen, means for the presentavvtion of signalsimpressed on said tube by said receiver along polar coordinates on saidscreen, means' coupled 'to said deilecting means for locating the polarorigin of said presentation substantially atl the periphery of saidscreen, means for sweeping the electron beam radially from said originsubstantially'entirely across said screen, and 'further means coupled tosaid deflecting meansv for angularly expanding said polar coordinatepresentation, whereby a given angle is represented by a larger angle onsaid screen.

CHALMERS W. SHERWIN. LAWRENCE H. JOHNSTON.

REFERENCES CITED The following references are of record in the ille otthis patent:

UNITED STATES PATENTS Number Name Date 2,189,549 Hershberger Feb. 6,1940 2,228,266 Gray Jan. 14,1941 2,231,929 Lyman Feb. 18,1941 2,241,809De Forest May 13, 1941 2,412,702 Woli! -s Dec. 17,1946 2,415,094. Hansenet al Feb. 4, 1947 2,421,747 Engelhardt June 10, 1947 2,422,361 MillerJune 17, 1947 FOREIGN PATENTS Number Country Date 108,556 AustraliaSept. 28, 1939

